The general construction design for an alumina reduction cell cathode comprises an outer open-top steel shell, several layers of high, intermediate and low temperature insulation refractories on the bottom of the steel shell and, in some instances, on the sidewalls of the steel shell, a layer of prebaked and/or monolithic rammed carbon on the bottom and sidewalls of the cell, a monolithic, prebaked carbon cathode on the floor of the cell, and busbars extending from the carbon cathode through the sidewalls of the cell for connection to an electrical system supplying the current necessary for reduction of alumina contained in the cell to aluminum.
An alumina reduction cell requires adequate insulation on the bottom and sidewalls of the cathode to limit heat losses from the steel shell during cell operation. Cryolitic salts and vapors, containing an excess of sodium fluoride, penetrate through the carbon bottom and sidewalls during operation of the cell over its normal four to six year lifespan, and chemically attack and degrade the insulation. As the insulation is degraded, it loses its effectiveness as a thermal insulation material and heat losses through the insulation increase. As a consequence, the cell voltage must be increased to maintain a stable thermal equilibrium in the cell. If the cell voltage is not increased, the temperature of the cryolite-alumina electrolyte decreases, resulting in an increase in anode effect frequency from a normal average of one anode effect per day per cell to about two to three anode effects per day per cell.
An increased anode effect frequency significantly decreases the productivity of the potline to which the cell is connected. First, the productivity of the cell experiencing the anode effect is reduced due to increased bath temperature and increased turbulence within the cell occurring during the anode effect. Additionally, the line amperage of all the reduction cells in the potline is affected. Alumina cells in a potline are connected in an electrical series. When an anode effect occurs in one of the cells, the line amperage typically decreases between about 3000 to 5000 amps, due to the high voltage, approximately 20 to 50 volts, on the cell having the anode effect, as opposed to the typical 5 to 7 volts of a normal cell. Thus, the productivity of all cells in the potline decreases during each anode effect.
Thus, as is readily apparent, increased heat losses from cathodes, as a result of degradation of the insulation by cryolitic salts, results in an increase in energy consumption and/or decrease in productivity of the cells.
The highly corrosive cryolitic salts and vapors penetrating the cathode can be stopped in one of two ways. The temperature isotherm directly above the insulation material may be kept sufficently low, typically below about 600.degree. C. to prevent any mobility of the salts below their freezing point. Alternatively, a vapor-proof barrier that will effectively resist the chemical attack of the cryolitic salts for the life of the cathode may be maintained in the cell.
In modern reduction cells, heat losses from the bottom and sides are reduced to conserve energy by adding additional layers of insulation and/or using insulation with lower thermal conductivities. This results in temperature isotherms directly above the insulation greater than about 800.degree. C., due to the reduction of heat flow through the insulation. Because of this higher surface temperature, the insulation will be attacked and degraded faster by the cryolitic salt vapors as the temperature isotherm at the surface of the insulation exceeds the freezing point of the salts, typically in the range of 700.degree. to 800.degree. C. Thus, reliance upon vapor barriers is the only viable alternative in modern alumina reduction cells for insulation protection.
Various materials have been used in the past to protect alumina reduction cell insulation material. For example, mild steel is often placed over the insulation material forming the bottom of the cell. While steel barriers are somewhat effective, they are themselves attacked and eroded by the cryolithic material, usually within about two to three years, and sooner if the carbon cathode develops cracks.
There are other disadvantages to be noted when employing steel as a barrier material. Increased steel thickness will gain only slightly increased barrier life, but at a substantial increased cost. Thus, the cost-benefit ratio of steel is poor. It is also difficult and expensive and to purchase a large, one-piece sheet of steel sufficient to cover the entire bottom surface of the cathode. At the same time, welding several smaller pieces of steel together will cause the composite sheet to warp, causing voids in the insulation.
Substituting stainless steel for mild steel does increase the barrier properties, but at a cost prohibitively high and with significant increased difficulty of welding.
Another approach used for protecting the floor insulation of a cell is a mortared layer of fire brick or tile. These tiles or bricks are joined with a high temperature mortar. While used extensively abroad, such barriers have not gained acceptance in the United States, due to the exceptionally high cost in increased construction time resulting from the brick laying process, both in materials and labor. Further, even when installed, there is a weak link in this system, namely, the mortar. The mortar does not have the same physical and chemical resistance as the bricks to the cryolitic salts. As a result, cryolitic salts and vapors eventually penetrate through the mortar, around the bricks, and attack the insulation.
Recently, it has been proposed to employ a layer of glass sandwiched between alumina silicate fiber blankets to form a thin chemical barrier against cryolitic salts, due to the formation of higher melting point compounds, such as napthalenes, etc. Although this concept appeared feasible during a one-year experiment, it has not proven successful in barring cryolitic salts and vapors for the full four to six year lifetime of a cell. It has been found that the higher melting point compounds will be attacked, dissolved and degraded by the highly corrosive cryolitic salts and that the overwhelming supply of semi-molten cryolitic salts and vapors attacks and corrodes the relatively thin glass layer. For example, in a typical alumina reduction cell, the cathode weight often doubles during the four to six year life of cell operation due to the absorption of cryolitic salts into the cathode lining. The relatively thin glass layers have been unable to withstand this quantity of corrosive material.
There is a need, therefore, for a vapor barrier to protect the insulation layers on the bottom of an electrolytic alumina reduction cell. There is also a need for a vapor barrier which may be employed on the sidewalls of a alumina reduction cell having insulated sidewalls. It is thus the primary objective of the present invention to provide such vapor barriers.